Heteroepitaxy is often used to create layered structures of different types of materials on substrates that are not lattice matched and otherwise would not form high quality epitaxial layers on the substrates. For instance, gallium nitride (GaN) does not form well on substrates made of sapphire (Al2O3) or silicon on sapphire (SOS) because of a relatively high level of lattice mismatch between the GaN and the sapphire or silicon on sapphire. The lattice mismatch often results in defects and dislocations in the GaN. To overcome these problems, epitaxial lateral overgrowth (ELOG) is used to form the GaN material, or other material, on another material, typically, through a channel or aperture in a dielectric material on a crystalline substrate.
An example of a conventional structure 100 formed through heteroepitaxy of an epitaxial lateral overgrowth (ELOG) material 110, such as, GaN, indium phosphide (InP), gallium arsenide (GaAs), etc., and a substrate 120 is shown in FIG. 1. As shown therein, a cross-sectional view of the structure 100 illustrates that a mask 130 is created on the substrate 120 using the dielectric material. The ELOG material 110 is shown without hash marks to more clearly show its position with respect to the mask 130.
The structure 100 is also depicted as including a channel 132, which is typically a few microns wide, in the dielectric mask 130. The ELOG material 110 is grown out of the channel 132 from seed material layer 120. More particularly, the ELOG material 110 grows from the channel 132 and spreads out of the channel 132 and across parts of the dielectric mask 130, as shown in FIG. 1. The dielectric material is used to form the mask 130 because it is able to withstand the relatively high temperatures that are required to grow the ELOG material 110 and also enables a suitable surface on which the ELOG material 110 laterally grows. As a comparison, conventional metals typically cannot be used as the mask 130 because the metals are prone to melting or forming alloys with the ELOG material at the temperatures required for the ELOG material to properly grow.
One problem with the ELOG material 110 formation is that as the ELOG material 110 grows out of the channel 132, defects 112 are often created in the transition from the vertical extension to the lateral overgrowth. However, material quality tends to increase as the layer spreads out over the dielectric mask 130. To avoid the defects 112, devices 140 are typically built upon wing portions 114 of the ELOG material 110, or the wing portions 114 form parts of the devices 140, which are generally laterally spaced from the defects 112. This configuration results in some undesirable characteristics in the structure 100.
For instance, the insulating properties of the dielectric mask 130 cause current crowding and thermal crowding because electrical current must flow along the wings 114 of the ELOG material 110 and then through the material 110 contained in the channel 132, as indicated by the arrows 142. In addition, heat generated in the devices 140 must also flow through this path to become dissipated through the substrate 120. This is not an ideal path for either the current or heat to flow because it constricts the heat and current flow and often results in overheating, which may impair the functioning of devices 140. In addition, the current crowding causes non-uniform current and the behavior of the devices 140 consequently suffers. The current and thermal crowding ultimately limits the flexibility of the devices that may be built using a dielectric mask and often requires additional heat dissipation solutions, which adds to the costs associated with implementing the structure 100, as well as the size of the structure 100.